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Healing of a hole in a carbon nanotube under electron irradiation in high-resolution transmission electron microscopy at room temperature is demonstrated using molecular dynamics simulations with t...

The electrical conductivity of the polyimide R-BAPB polymer filled with single-wall carbon nanotubes (CNT) with chirality (5,5) is modeled using a multi-scale approach. The modeling starts with molecular dynamics simulations of time-dependent fluctuating atomic configurations of polymer filled CNTs junctions. Then the atomic positions obtained in the first step are used to perform fully first-principles microscopic calculations of the CNTs junctions contact resistances using the Green's function based quantum transport technique. And finally, those contact resistances are supplied as an input to a statistical calculation of a CNTs ensemble conductivity using a Monte Carlo percolation model. The results of the first-principles calculations show a very strong dependence of the polymer filled CNTs junctions contact resistance on the geometry of CNTs junctions, including an angle $\varphi$ between nanotubes axes and the positions of polymer atoms around CNTs. Incorporating into the percolation model this strong dependence as well as CNTs agglomeration, pushed the calculated values of electrical conductivity just above the percolation threshold below 0.01 S/m, which is within the experimental range for composites with various base polymers. Possible mechanisms for further reduction of composites conductivity are discussed., Comment: 12 pages, 9 figures

In this paper, we develop a coarse grained numerical model for the simulation of a self-organization process for a system of carbon nanotubes under the applied electric field. The model describes the polarization of nanotubes in the system with an electric field and also includes the Van der Waals interaction between nanotubes. We develop an iterative computation algorithm for particle charges in the nanotube, providing a significant speedup of the computation. Another advantage of this algorithm is the better scaling of the computation time as a function of the system size. The results of the application of this model to computing the self-organization process of the dynamics of carbon nanotubes are demonstrated.

The elastic properties of a flat graphene layer calculated using the classical empirical Tersoff, Brenner, AIREBO, PPBE-G, and LCBOP potentials have been compared. It is shown that, although the popular Brenner and AIREBO potentials have been developed formally taking into account the elastic properties of graphene, they give significant discrepancies in the values of Young’s modulus and Poisson’s ratio. Among the potentials under consideration, the LCBOP potential yields the values of these parameters that are closest to experimental data and results of ab initio calculations in the limit of zero elongation. For the quantitative simulation of mechanical phenomena in graphene-based systems, the potential parameters should be fitted to reproduce elastic properties of graphene completely taking into account system deformations and dependences of these constants on the elongation.

Novel logarithmic time algorithm is proposed for determining the order of chemical bonds from known valences of the atoms. The algorithm has the order of complexity N · log(N) and is applicable to polycyclic compounds containing a combination of the cycles of any size and the atoms with the valences ≤4 in cycle nodes. The algorithm is applicable to structures containing triple and cumulene bonds in the cycles. It was tested for graphene, C[12,12] nanotubes, graphyne-GY1, graphyne-GY7, graphyne-like nanotube, fullerenes C20, C60, C70, C80, C82 and their aza-analogs, polypentadienes, as well as for porphine. Determining the order of bonds in graphene, graphynes or nanotubes, containing >107 atoms, took less than 2 min on a personal computer. For the compounds containing aromatic cycles with an odd number of atoms, this algorithm becomes probabilistic and successful determination of bond orders is not guaranteed. However, the probability of successful determination of bond order is significant, and Kekule structures, if they existed, were generated for all studied fullerenes and their aza-analogs.

Molecular dynamics simulations show that a graphene nanoribbon with alternating regions which are one and three hexagons wide can transform into a hybrid 1D nanoobject with alternating double chains and polycyclic regions under electron irradiation in HRTEM. A scheme of synthesis of such a nanoribbon using Ullmann coupling and dehydrogenation reactions is proposed. The reactive REBO-1990EVC potential is adapted for simulations of carbon–hydrogen systems and is used in combination with the CompuTEM algorithm for modeling of electron irradiation effects. The atomistic mechanism of formation of the new hybrid 1D nanoobject is found to be the following. Firstly hydrogen is removed by electron impacts. Then spontaneous breaking of bonds between carbon atoms leads to the decomposition of narrow regions of the graphene nanoribbon into double chains. Simultaneously, thermally activated growth of polycyclic regions occurs. Density functional theory calculations give barriers along the growth path of polycyclic regions consistent with this mechanism. The electronic properties of the new 1D nanoobject are shown to be strongly affected by the edge magnetism and make this nanostructure promising for nanoelectronic and spintronic applications. The synthesis of the 1D nanoobject proposed here can be considered as an example of the general three-stage strategy of production of nanoobjects and macromolecules: (1) precursors are synthesized using a traditional chemical method, (2) precursors are placed in HRTEM with the electron energy that is sufficient only to remove hydrogen atoms, and (3) as a result of hydrogen removal, the precursors become unstable or metastable and transform into new nanoobjects or macromolecules., A. S. S., A. M. P. and A. A. K. acknowledge the Russian Foundation of Basic Research (Grants 18-02-00985 and 20-52-00035). S. V. R. and N. A. P. acknowledge the Belarusian Republican Foundation for Fundamental Research (Grant No. F20R-301) and Belarusian National Research Program ‘‘Convergence-2020’’. This work has been carried out using the computing resources of the federal collective usage center Complex for Simulation and Data Processing for Mega-science Facilities at NRC ‘‘Kurchatov Institute’’, http://ckp.nrcki.ru/ and was supported by the Research Center ‘‘Kurchatov Institute (order No. 1569 of July 16, 2019). D. G. O. has received funding from the Spanish Agencia Estatal de Investigacion (Grant No. PID2019-107338RB-C63).

The potential energy surface (PES) of interlayer interaction of twisted bilayer graphene with vacancies in one of the layers is investigated via density functional theory (DFT) calculations with van der Waals corrections. These calculations give a non-negligible magnitude of PES corrugation of 28 meV per vacancy and barriers for relative sliding of the layers of 7 - 8 meV per vacancy for the moir\'e pattern with coprime indices (2,1) (twist angle 21.8$^\circ$). At the same time, using the semiempirical potential fitted to the DFT results, we confirm that twisted bilayer graphene without defects exhibits superlubricity for the same moir\'e pattern and the magnitude of PES corrugation for the infinite bilayer is below the calculation accuracy. Our results imply that atomic-scale defects restrict the superlubricity of 2D layers and can determine static and dynamic tribological properties of these layers in a superlubric state. We also analyze computationally cheap approaches that can be used for modeling of tribological behavior of large-scale systems with defects. The adequacy of using state-of-the-art semiempirical potentials for interlayer interaction and approximations based on the first spatial Fourier harmonics for the description of interaction between graphene layers with defects is discussed., Comment: 11 pages, 4 figures

We propose a hypothesis that the potential energy surface (PES) of interlayer interaction in diverse 2D materials can be universally described by the first spatial Fourier harmonics. This statement (checked previously for the interactions between graphene and hexagonal boron nitride layers in different combinations) is verified in the present paper for the case of hydrofluorinated graphene (HFG) bilayer with hydrogen bonding between fluorine and hydrogen at the interlayer interface. The PES for HFG bilayer is obtained through density functional theory calculations with van der Waals corrections. An analytical expression based on the first Fourier harmonics describing the PES which corresponds to the symmetry of HFG layers is derived. It is found that the calculated PES can be described by the first Fourier harmonics with the accuracy of 3\% relative to the PES corrugation. The shear mode frequency, shear modulus and barrier for relative rotation of the layers to incommensurate states of HFG bilayer are estimated. Additionally it is shown that HFG bilayer is stable relative to the formation of HF molecules as a result of chemical reactions between the layers., Comment: 12 pages, 3 figures

Plasma treatment of porous polymeric materials is a promising method for creating new materials that can be used for different tasks, including in medicine, for the development of new types of biocompatible and biodegradable polymeric materials. This paper is devoted to the study of plasma treatment of porous polylactide to clarify discharge conditions and improve the effectiveness of the treatment process. Experiments were conducted to determine conditions for the ignition of a discharge and its electrical parameters in samples of porous polylactide at different air pressures. An experimental study of the degree of processing (hydrophilization) of the cavities of polylactide samples was carried out depending on processing time, gas pressure, and the corresponding input energy of the discharge. An infrared spectroscopic study of the treated and untreated samples was conducted, and the impact of treatment on the functional groups on the polymer surface is shown. The effect of electret properties of polylactide on the discharge ignition in its cavities is demonstrated.

Despite the high potential of endohedral metallofullerenes (EMFs) for application in biology, medicine and molecular electronics, and recent efforts in EMF synthesis, the variety of EMFs accessible by conventional synthetic methods remains limited and does not include, for example, EMFs of late transition metals. We propose a method in which EMF formation is initiated by electron irradiation in aberration-corrected high-resolution transmission electron spectroscopy (AC-HRTEM) of a metal cluster surrounded by amorphous carbon inside a carbon nanotube serving as a nanoreactor and apply this method for synthesis of nickel EMFs. The use of AC-HRTEM makes it possible not only to synthesize new, previously unattainable nanoobjects but also to study in situ the mechanism of structural transformations. Molecular dynamics simulations using the state-of-the-art approach for modeling the effect of electron irradiation are performed to rationalize the experimental observations and to link the observed processes with conditions of bulk EMF synthesis., A.S.S., I.V.L., A.A.K., and A.M.P. acknowledge the Russian Foundation of Basic Research (14-02-00739-a). I.V.L. acknowledges the financial support from Grupos Consolidados UPV/EHU del Gobierno Vasco (IT578-13) and EU-H2020 project “MOSTOPHOS” (no. 646259). A.N.K., T.W.C., and R.L.M. acknowledge the ERC and EPSRC for financial support, and the Nanoscale & Microscale Research Centre (nmRC). T.Z., J.B., and U.K. gratefully acknowledge the funding by the DFG (German Research Foundation) and the Ministry of Science, Research and the Arts (MWK) of Baden Wuerttemberg in the framework of the SALVE (Sub Angstrom Low-Voltage Electron Microscopy) project.

The capacity of popular classical interatomic potentials to describe elastic properties of graphene is tested. The Tersoff potential, Brenner reactive bond-order potentials REBO-1990, REBO-2000, REBO-2002 and AIREBO as well as LCBOP, PPBE-G, ReaxFF-CHO and ReaxFF-C2013 are considered. Linear and non-linear elastic response of graphene under uniaxial stretching is investigated by static energy calculations. The Young's modulus, Poisson's ratio and high-order elastic moduli are verified against the reference data available from experimental measurements and ab initio studies. The density functional theory calculations are performed to complement the reference data on the effective Young's modulus and Poisson's ratio at small but finite elongations. It is observed that for all the potentials considered, the elastic energy deviates remarkably from the simple quadratic dependence already at elongations of several percent. Nevertheless, LCBOP provides the results consistent with the reference data and thus realistically describes in-plane deformations of graphene. Reasonable agreement is also observed for the computationally cheap PPBE-G potential. REBO-2000, AIREBO and REBO-2002 give a strongly non-linear elastic response with a wrong sign of the third-order elastic modulus and the corresponding results are very far from the reference data. The ReaxFF potentials drastically overestimate the Poisson's ratio. Furthermore, ReaxFF-C2013 shows a number of numerical artefacts at finite elongations. The bending rigidity of graphene is also obtained by static energy calculations for large-diameter carbon nanotubes. The best agreement with the experimental and ab initio data in this case is achieved using the REBO-2000, REBO-2002 and ReaxFF potentials. Therefore, none of the considered potentials adequately describes both in-plane and out-of-plane deformations of graphene., AMP and AAK acknowledge the Russian Foundation for Basic Research, Russia (Grant 18-02-00985). IVL acknowledges Grupos Consolidados del Gobierno Vasco (IT-578-13).

The electrical properties of a polymer composite with carbon nanotube additives have been analyzed. The state of the system near the percolation threshold, when charge is transferred along a single percolation path, has been considered. For this state, the current–voltage characteristics of a percolation chain made up of carbon nanotubes have been calculated under the assumption that the contact resistance between neighboring nanotubes is much higher than the intrinsic resistance of the nanotubes. According to recent data, the distance between neighboring (contacting) nanotubes has been assumed to be randomly distributed. It has been shown that, under the given conditions, the current–voltage characteristic is essentially nonlinear. This indicates the nonohmic conductivity of the composites. The dependence of the current–voltage characteristic on the spread of the contact distribution over distances has been discussed.

Theoretical analysis of optimization options for the properties of CdTe absorber layer is an important task for increasing the efficiency of CdTe/CdS heterojunction based thin-film solar cells. Properties of the materials (e.g. the density of free carriers) often depend essentially on the parameters of the deposition process and subsequent treatment which determine the defect composition of the material. In this work a model based on the lattice kinetic Monte-Carlo method is developed to describe the process of CdTe deposition as a function of temperature and Cd and Te fluxes. To determine the effect of the treatment conditions on CdTe conductivity, we developed a quasichemical model based on the electrical neutrality equation for point defect concentrations that are described by defect formation reaction constants. Parameters obtained from the first-principles density functional calculations were used for developing the models. The developed deposition model correctly describes the transition from evaporation to precipitation as well as the increased evaporation rates in excess of Cd. To explain the observed electrical properties of CdTe after Cl-treatment, we complemented the quasichemical defect model by a deep acceptor complex defect that allowed us to describe both the high-temperature dependence of conductivity on the Cd pressure and the dependence of resistivity on Cl concentration at room temperature.

Single-layer and bilayer carbon and hexagonal boron nitride nanoscrolls as well as nanoscrolls made of bilayer graphene/hexagonal boron nitride heterostructure are considered. Structures of stable states of the corresponding nanoscrolls prepared by rolling single-layer and bilayer rectangular nanoribbons are obtained based on the analytical model and numerical calculations. The lengths of nanoribbons for which stable and energetically favorable nanoscrolls are possible are determined. Barriers to rolling of single-layer and bilayer nanoribbons into nanoscrolls and barriers to nanoscroll unrolling are calculated. Based on the calculated barriers nanoscroll lifetimes in the stable state are estimated. Elastic constants for bending of graphene and hexagonal boron nitride layers used in the model are found by density functional theory calculations., AL, AP, and YL acknowledge the Russian Foundation of Basic Research (16-52-00181). IL acknowledges the financial support from Grupos Consolidados UPV/EHU del Gobierno Vasco (IT578-13) and EU-H2020 project “MOSTOPHOS” (n. 646259). AS, NP, and SV acknowledge the Belarusian Republican Foundation for Fundamental Research (grant No. F16R107) and Belarusian National Research Program “Convergence”.

The method for production of atomic chains by heating of graphene nanoribbons (GNRs) is proposed and studied by molecular dynamics simulations. The Brenner potential is revised to adequately describe formation of atomic chains, edges and vacancy migration in graphene. A fundamentally different behaviour is observed for zigzag-edge GNRs with 3 and 4 atomic rows (3 and 4-ZGNRs) at 2500 K: formation of triple, double and single carbon chains with the length of hundreds of atoms in 3-ZGNRs and edge reconstruction with only short chains and GNR width reduction in 4-ZGNRs. The chain formation mechanism in 3-ZGNRs is revealed by analysis of bond reorganization reactions and is based on the interplay of two processes. The first one is breaking of bonds between 3 zigzag atomic rows leading to triple chain formation. The second one is bond breaking within the same zigzag atomic row, which occurs predominantly through generation of pentagons with subsequent bond breaking in pentagons and results in single or double chain formation. The DFT calculations of the barriers for relevant reactions are consistent with the mechanism proposed. The possibility of chain-based nanoelectronic devices with a controllable number of chains is discussed., ASS, AMP and AAK acknowledge the Russian Foundation for Basic Research, Russia (Grant 18-02-00985). IVL acknowledges Grupos Consolidados del Gobierno Vasco, Spain (IT-578-13) and EU-H2020 project "MOSTOPHOS" (n. 646259).

The telescopic contact between graphene layers with a dielectric spacer is considered as a new type of graphene-based nanoelectronic devices. The tunneling current through the contacts with and without an argon spacer is calculated as a function of the overlap length, stacking of the graphene layers and voltage applied using non-equilibrium Green function formalism. A negative differential resistance (similar to semiconductor tunnel diode) is found with the peak to valley ratio up to 10 and up to 2 for the contacts without any spacer and with the argon spacer, respectively. The capacitance of the contacts between the graphene layers with the argon spacer is calculated as a function of temperature taking into account the quantum contribution. The related RC time constant is estimated to be about 3 ps, which allows elaboration of fast-response nanoelectronic devices. The possibility of application of the contacts as memory cells is discussed.

This paper reviews research into the electrical properties that are imparted to composite materials by introducing carbon nanotubes (CNTs) into their polymer matrices. Due to the large aspect ratio of CNTs, even a small amount of doping (at a level of 0.01 - 0.1%) is enough to increase the conductivity of the material by more than ten orders of magnitude, thus changing it from an insulator to a conductor. At low doping, charge transfer is of a percolation nature in the sense that nanotubes that are in contact with each other form conducting channels in the material. Importantly, the conductivity has a threshold nature, so that the conduction jump occurs upon an arbitrarily small increase in a doping level above the critical value. This paper summarizes experimental data on the position of the percolation threshold and the maximum magnitude of the conductivity for composites obtained using various polymer types and a variety of CNT geometries. Factors affecting the electrical characteristics of composites produced by distinct methods are analyzed. Methods for and basic results obtained from the simulation of the percolation conductivity of CNT-doped composites are discussed. Particular attention is given to contact phenomena that occur at adjacent nanotube boundaries and which determine the conductivity of CNT-doped composites.

Incomplete stacking dislocations are predicted to form at edges of the shorter upper layer in two-dimensional hexagonal bilayers upon stretching the longer bottom layer. A concept of the edge Burgers vector is introduced to describe such dislocations by analogy with the Burgers vector of standard bulk dislocations. Analytical solutions for the structure and energy of edge stacking dislocations in bilayer graphene are obtained depending on the magnitude of elongation and angles between the edge Burgers vector, direction of elongation and edge. The barrier for penetration of stacking dislocations inside the bilayer is estimated. The possibilities to measure the barrier to relative motion of graphene layers and strain of graphene on a substrate by observation of edge stacking dislocations are discussed., The authors acknowledge the Russian Foundation of Basic Research (14-02-00739-a). IL acknowledges the financial support from Grupos Consolidados UPV/EHU del Gobierno Vasco (IT578-13) and H2020-NMP-2014 project “MOSTOPHOS” (n. 646259).

Exchange-correlation functionals with corrections for van der Waals interactions (PBE-D2, PBE-D3, PBE-D3(BJ), PBE-TS, optPBE-vdW and vdW-DF2) are tested for graphene and hexagonal boron nitride, both in the form of bulk and bilayer. The characteristics of the potential energy surface, such as the barrier to relative sliding of the layers and magnitude of corrugation, and physically measurable properties associated with relative in-plane and out-of-plane motion of the layers including the shear modulus and modulus for axial compression, shear mode frequency and frequency of out-of-plane vibrations are considered. The PBE-D3(BJ) functional gives the best results for the stackings of hexagonal boron nitride and graphite that are known to be ground-state from the experimental studies. However, it fails to describe the order of metastable states of boron nitride in energy. The PBE-D3 and vdW-DF2 functionals, which reproduce this order correctly, are identified as the optimal choice for general studies. The vdW-DF2 functional is preferred for evaluation of the modulus for axial compression and frequency of out-of-plane vibrations, while the PBE-D3 functional is somewhat more accurate in calculations of the shear modulus and shear mode frequency. The best description of the latter properties, however, is achieved also using the vdW-DF2 functional combined with consideration of the experimental interlayer distance. In the specific case of graphene, the PBE-D2 functional works very well and can be further improved by adjustment of the parameters., AL and AP acknowledge the Russian Foundation for Basic Research (Grant 16-52-00181). IL acknowledges the financial support from Grupos Consolidados UPV/EHU del Gobierno Vasco (IT578-13) and EU-H2020 project “MOSTOPHOS” (n. 646259).

The interlayer interaction in graphene/boron-nitride heterostructures is studied using density functional theory calculations with the correction for van der Waals interactions. It is shown that the use of the experimental interlayer distance allows one to describe the potential energy surface at the level of more accurate but expensive computational methods. On the other hand, it is also demonstrated that the dependence of the interlayer interaction energy on the relative in-plane position of the layers can be fitted with high accuracy by a simple expression determined by the system symmetry. The use of only two independent parameters in such an approximation suggests that various physical properties of flat graphene/boron-nitride systems are interrelated and can be expressed through these two parameters. Here we estimate some of the corresponding physical properties that can be accessed experimentally, including the correction to the period of the moiré superstructure for the highly incommensurate ground state of the graphene/boron-nitride bilayer coming from the interlayer interaction, the width of stacking dislocations in slightly incommensurate systems of boron nitride on stretched graphene, and shear mode frequencies for commensurate graphene/boron-nitride systems, such as a flake on a layer. We propose that the commensurate-incommensurate phase transition can be observed in boron nitride on stretched graphene and that experimental measurements of the corresponding critical strain can be also used to gain insight into graphene/boron-nitride interactions., A.V.L. and A.M.P. acknowledge the Russian Foundation of Basic Research (Grant No. 16-52-00181) and computational time on the Multipurpose Computing Complex NRC “Kurchatov Institute”. I.V.L. acknowledges financial support from Grupos Consolidados UPV/EHU del Gobierno Vasco (Grant No. IT578-13) and H2020-NMP-2014 Project “MOSTOPHOS” (No. 646259).

Solar flares are observed and classified according to their intensity measured with the GOES X-ray Sensors. We show that the duration of a flare, as measured by the full width at half maximum (FWHM) in GOES is not related to the size of the flare as measured by GOES intensity. The durations of X-class flares range from a few minutes to a few hours, and the same is true of M- and C-class flares. In this work, we therefore examine the statistical relationships between the basic properties of flares -- temperature, emission measure, energy, etc. -- in comparison to both their size and duration. We find that the size of the flare is directly related to all of these basic properties, as previously found by many authors. The duration is not so clear. When examining the whole data set, the duration appears to be independent of all of these properties. In larger flares, however, there are direct correlations between the GOES FWHM and magnetic reconnection flux and ribbon area. We discuss the possible explanations, finding that this discrepancy may be due to large uncertainties in small flares, though we cannot rule out the possibility that the driving physical processes are different in smaller flares than larger ones. We discuss the implications of this result and how it relates to the magnetic reconnection process that releases energy in flares., ApJ, in press

Large-amplitude longitudinal (LAL) prominence oscillations consist of periodic mass motions along a filament axis. The oscillations appear to be triggered by an energetic event, such as a microflare, subflare, or small C-class flare, close to one end of the filament. Observations reveal speeds of several tens to 100 km/s, periods of order 1 hr, damping times of a few periods, and displacements equal to a significant fraction of the prominence length. We have developed a theoretical model to explain the restoring force and the damping mechanism. Our model demonstrates that the main restoring force is the projected gravity in the flux tube dips where the threads oscillate. Although the period is independent of the tube length and the constantly growing mass, the motions are strongly damped by the steady accretion of mass onto the threads. We conclude that the LAL movements represent a collective oscillation of a large number of cool, dense threads moving along dipped flux tubes, triggered by a nearby energetic event. Our model yields a powerful seismological method for constraining the coronal magnetic field strength and radius of curvature at the thread locations.

The possibility to control the commensurability and distance between graphene layers separated by a dielectric spacer is considered by the example of a heterostructure consisting of double-layer graphene separated by atomic layers of argon. Van der Waals-corrected density functional theory (DFT-D) is applied to study structural, energetic, and tribological characteristics of this heterostructure. It is found that, in the ground state, monolayer and bilayer argon spacers are incommensurate with the graphene layers, whereas sub-monolayer argon spacers which are commensurate with the graphene layers can exist only as metastable states. The calculations show that this incommensurability provides negligibly small static friction for relative motion of argon-separated graphene layers; therefore, such a heterostructure holds great promise for use in nanoelectromechanical systems. A scheme and operational principles of the nanorelay based on the revealed tribological properties of the heterostructure are proposed.

The possibility of designing nanoelectromechanical systems (NEMS) based on relative motion or vibrations of graphene layers is analyzed. Ab initio and empirical calculations of the potential relief of interlayer interaction energy in bilayer graphene are performed. A new potential based on the density functional theory calculations with the dispersion correction is developed to reliably reproduce the potential relief of interlayer interaction energy in bilayer graphene. Telescopic oscillations and small relative vibrations of graphene layers are investigated using molecular dynamics simulations. It is shown that these vibrations are characterized with small Q-factor values. The perspectives of nanoelectromechanical systems based on relative motion or vibrations of graphene layers are discussed., Comment: 19 pages, 4 figures

The problems of the electric field action on carbon nanotubes (CNTs) during their growth and under the electron field emission conditions are considered. The relations determining the growth rate of an extended structure under the action of the electric field are established. The relation connecting the angle of orientation of a CNT inclined to the substrate surface and the applied electric field is used for computing current-voltage characteristics of the cathode consisting of inclined CNTs. The degree of deviation of these characteristics from the Fowler-Nordheim classic dependence is determined, on the one hand, by the parameters characterizing the CNT spread over the angles of inclination and, on the other hand, by the value of the Young modulus characterizing the bending stiffness of a nanotube. It is shown that in zero external electric field, a certain effect on the CNT orientation can be produced by the CNT potential relative to the substrate, which is due to the effect of the contact potential difference.

We investigate the influence of hydrogen on carbon nanotube (CNT) growth in thermal catalytic chemical vapor deposition. Kinetic calculations of gas-phase transformations of hydrocarbons show that hydrogen interacts with gaseous carbon precursors, resulting in modification of the carbon supply rate to the catalyst particle. A surface-kinetic model of CNT growth is developed to study adsorption and decomposition kinetics of precursors on Ni catalyst particles. The detailed surface kinetics of carbon precursors and transport of carbon atoms through the catalyst particle are described in the framework of the surface/bulk site formalism, with the parameters of the reactions determined on the basis of first-principles calculations for Ni (1 1 1) and (1 1 3) surfaces. Using this model, different regimes of CNT growth, with and without hydrogen in the system, are analyzed. Hydrogen is shown to enhance desorption of hydrocarbons, leading to a decrease of the surface coverage and effective carbon supply rate.

A new type of ultrahigh frequency resonator based on the relative vibrations of carbon nanotubes walls is proposed. Microcanonical molecular dynamics simulations performed for a model resonator based on the (9,0)@(18,0) double-walled carbon nanotube with the movable outer wall give the value of frequency equal to 130 GHz and the quality factor up to 500. Possible applications of the resonator are discussed. The estimated mass sensitivity of the mass sensor based on the proposed resonator reaches the atomic mass limit at liquid helium temperature. The possibility of resonator based on relative vibrations of graphene layers is also considered.

We have demonstrated that the ability of fullerenes to form clusters is essential for the filling of single-walled carbon nanotubes in solution. In solutions where C60 exists in the form of discrete solvated molecules (e.g., in CS2) no fullerene encapsulation in nanotubes takes place, as the large molar excess of solvent compared to solute prohibits C60 from entering the nanotubes. However, in solutions containing large clusters of C60 (e.g., in n-hexane) nanotubes become densely filled with fullerene molecules despite the large excess of solvent. The interactions between carbon nanotubes and fullerene clusters provide an efficient transport of C60 into nanotubes that avoids the detrimental effects of the solvent molecules. This new mechanism provides the first rational explanation of experiments involving nanotube filling with guest-molecules in solution.

The structure and elastic properties of (5,5) and (10,10) nanotubes, as well as barriers for relative rotation of the walls and their relative sliding along the axis in a double-walled (5,5)@(10,10) carbon nanotube, are calculated using the density functional method. The results of these calculations are the basis for estimating the following physical quantities: shear strengths and diffusion coefficients for relative sliding along the axis and rotation of the walls, as well as frequencies of relative rotational and translational oscillations of the walls. The commensurability-incommensurability phase transition is analyzed. The length of the incommensurability defect is estimated on the basis of ab initio calculations. It is proposed that (5,5)@(10,10) double-walled carbon nanotube be used as a plain bearing. The possibility of experimental verification of the results is discussed., Comment: 14 pages

A new method is proposed for controlling the motion of nanoelectromechanical systems based on carbon nanotubes. In this method, a single-walled nanotube acquires an electric dipole moment owing to the chemical adsorption of atoms or molecules at open ends of the nanotube. The electric dipole moments of carbon nanotubes with chemically modified ends are calculated by the molecular orbital method. These nanotubes can be set in motion under the effect of a nonuniform electric field. The possibility of controlling the motion of nanoelectromechanical systems with the proposed method is demonstrated using a nanotube-based gigahertz oscillator as an example. The operating characteristics of the gigahertz oscillator are analyzed, and its operation is simulated by the molecular dynamics method. The controlling parameters and characteristics corresponding to the controlled operating conditions at a constant frequency for the system under investigation are determined.

According to experimental investigations, carbon nanotubes and fullerenes can be formed from graphite sheets. We present here the results of the molecular dynamics simulation of the folding of a single graphite sheet. The role of defects in this process is examined. Furthermore, the possible ways of reducing the number of defects in an imperfect fullerene, such as Stone–Wales rearrangements and the addition of carbon atoms and dimers, are investigated. Based on these investigations, we estimated the temperature range within which perfect nanostructures can be obtained.

The kinetic lattice Monte Carlo method for film growth simulation without taking crystallization into account was applied to study the roughness of the HfO2 film grown by atomic layer deposition at 100–500°C from HfCl4 and H2O. The calculations were performed using a simplified kinetic mechanism of the growth of HfO2 films obtained by reducing the detailed kinetic mechanism developed earlier. Ab initio quantum-chemical calculations were performed to determine the kinetic parameters of diffusion processes on the surface of hafnium oxide that could influence film roughness. Because of the special features of atomic layer deposition, the rate of film growth and film roughness were finite even if surface relaxation was ignored. It was found that, irrespective of the temperature, the diffusion of hydrogen and adsorbed HfCl4 complexes did not change the profile of the growing film and only insignificantly increased the mean rate of growth. The results obtained were also qualitatively applicable to zirconium dioxide at fairly low (≤100°C) temperatures in the absence of crystallization.

arXiv.-- et al., The cutting of single-walled carbon nanotubes by an 80 keV electron beam catalyzed by nickel clusters is imaged in situ using aberration-corrected high-resolution transmission electron microscopy. Extensive molecular dynamics simulations within the CompuTEM approach provide insight into the mechanism of this process and demonstrate that the combination of irradiation and the nickel catalyst is crucial for the cutting process to take place. The atomistic mechanism of cutting is revealed by a detailed analysis of irradiation-induced reactions of bond reorganization and atom ejection in the vicinity of the nickel cluster, showing a highly complex interplay of different chemical transformations catalysed by the metal cluster. One of the most prevalent pathways includes three consecutive stages: formation of polyyne carbon chains from the carbon nanotube, dissociation of the carbon chains into single and pairs of adatoms adsorbed on the nickel cluster, and ejection of these adatoms leading to the cutting of the nanotube. Significant variations in the atom ejection rate are discovered depending on the process stage and nanotube diameter. The revealed mechanism and kinetic characteristics of the cutting process provide fundamental knowledge for the development of new methodologies for control and manipulation of carbon structures at the nanoscale., AMP, IVL and AAK acknowledge the Russian Foundation of Basic Research (14-02-00739-a). IVL acknowledges support by the Marie Curie International Incoming Fellowship within the 7th European Community Framework Programme (Grant Agreement PIIF-GA-2012-326435 RespSpatDisp), Grupos Consolidados del Gobierno Vasco (IT-578-13). TWC and ANK are supported by the European Research Council (ERC) and the Engineering & Physical Sciences Research Council (EPSRC).

The interaction and tunneling conductance between oppositely located ends of coaxial carbon nanotubes are studied by the example of two (11,11) nanotubes with open ends terminated by hydrogen atoms. The Green function formalism is applied to determine the tunneling current through the nanotube ends as a function of the distance between the ends, relative orientation of the nanotubes and voltage applied. The energy favorable configuration of the coaxial nanotubes is obtained by the analysis of their interaction energy at different distances between the nanotube ends and angles of their relative rotation. Using these calculations, a general scheme of the force sensor based on the interaction between ends of coaxial nanotubes is proposed and the relation between the tunneling conductance and measured force is established for the considered nanotubes. The operational characteristics of this device as a magnetic field sensor based on measurements of the magnetic force acting on the coaxial nanotubes filled with magnetic endofullerenes are estimated., Comment: 9 pages, 8 figures

arXiv.-- et al., A way to produce new metal-carbon nanoobjects by transformation of a graphene flake with an attached transition metal cluster under electron irradiation is proposed. The transformation process is investigated by molecular dynamics simulations by the example of a graphene flake with a nickel cluster. The parameters of the nickel-carbon potential (I. V. Lebedeva et al., J. Phys. Chem. C, 2012, 116, 6572) are modified to improve the description of the balance between the fullerene elastic energy and graphene edge energies in this process. The metal-carbon nanoobjects formed are found to range from heterofullerenes with a metal patch to particles consisting of closed fullerene and metal clusters linked by chemical bonds. The atomic-scale transformation mechanism is revealed by the local structure analysis. The average time of formation of nanoobjects and their lifetime under electron irradiation are estimated for the experimental conditions of high-resolution transmission electron microscopy (HRTEM). The sequence of images of nanostructure evolution with time during its observation by HRTEM is also modelled. Furthermore, the possibility of batch production of studied metal-carbon nanoobjects and solids based on these nanoobjects is discussed., AS, IL, AK and AP acknowledges Russian Foundation of Basic Research (14-02-00739-a). AP acknowledges Samsung Global Research Outreach Program. IL acknowledges support from the Marie Curie International Incoming Fellowship within the 7th European Community Framework Programme (Grant Agreement PIIF-GA-2012-326435 RespSpatDisp), Grupos Consolidados del Gobierno Vasco (IT-578-13) and the computational time on the Supercomputing Center of Lomonosov Moscow State University and the Multipurpose Computing Complex NRC “Kurchatov Institute.” EB acknowledges EPSRC Career Acceleration Fellowship, New Directions for EPSRC Research Leaders Award (EP/G005060), and ERC Starting Grant for financial support.

Nous mesurons la resonance magnetique nucleaire du 43 Ca en fonction de la temperature dans l'etat normal du supraconducteur a haute temperature (Ca x La 1-x )(Ba 1.75-x La 0,25+x )Cu 3 O y . Les echantillons sont choisis de maniere a comparer l'effet d'une modification des teneurs en calcium et en oxygene dans le regime sous-dope. Nous determinons les parametres quadripolaires et le deplacement de Knight (KS). Enfin, nous mesurons la susceptibilite magnetique macroscopique et l'utilisons pour estimer le champ hyperfin du 43 Ca. La variation de KS lors d'une augmentation de la teneur en calcium ne montre pas la signature d'une augmentation du niveau de dopage, contrairement a ce que suggere la variation des proprietes macroscopiques.

We report an unusual set of observations of waves in a large prominence pillar which consist of pulses propagating perpendicular to the prominence magnetic field. We observe a huge quiescent prominence with the Solar Dynamics Observatory (SDO) Atmospheric Imaging Assembly (AIA) in EUV on 2012 October 10 and only a part of it, the pillar, which is a foot or barb of the prominence, with the Hinode Solar Optical Telescope (SOT) (in Ca II and H\alpha lines), Sac Peak (in H\alpha, H\beta\ and Na-D lines), THEMIS ("T\'elescope H\'eliographique pour l' Etude du Magn\'etisme et des Instabilit\'es Solaires") with the MTR (MulTi-Raies) spectropolarimeter (in He D_3 line). The THEMIS/MTR data indicates that the magnetic field in the pillar is essentially horizontal and the observations in the optical domain show a large number of horizontally aligned features on a much smaller scale than the pillar as a whole. The data is consistent with a model of cool prominence plasma trapped in the dips of horizontal field lines. The SOT and Sac Peak data over the 4 hour observing period show vertical oscillations appearing as wave pulses. These pulses, which include a Doppler signature, move vertically, perpendicular to the field direction, along thin quasi-vertical columns in the much broader pillar. The pulses have a velocity of propagation of about 10 km/s, a period about 300 sec, and a wavelength around 2000 km. We interpret these waves in terms of fast magneto-sonic waves and discuss possible wave drivers., Comment: Accepted for publication in The Astrophysical Journal

A new concept of an electromechanical nanodynamometer based on the relative displacement of layers of bilayer graphene is proposed. In this nanodynamometer, force acting on one of the graphene layers causes the relative displacement of this layer and related change of conductance between the layers. Such a force can be determined by measurements of the tunneling conductance between the layers. Dependences of the interlayer interaction energy and the conductance between the graphene layers on their relative position are calculated within the first-principles approach corrected for van der Waals interactions and the Bardeen method, respectively. The characteristics of the nanodynamometer are determined and its possible applications are discussed., 5 pages, 4 figures

Interwall interaction energies of double-walled nanotubes (DWNTs) with long inner and short outer walls are calculated as functions of coordinates describing relative rotation and displacement of the walls using van der Waals corrected density functional theory. The magnitude of corrugation and the shape of the potential energy relief are found to be very sensitive to changes of the shorter wall length at subnanometer scale and atomic structure of the edges if at least one of the walls is chiral. Threshold forces required to start relative motion of the short walls and temperatures at which the transition between diffusive and free motion of the short walls takes place are estimated. The edges are also shown to provide a considerable contribution to the barrier to relative rotation of commensurate nonchiral walls. For such walls, temperatures of orientational melting, i.e. the crossover from rotational diffusion to free relative rotation, are estimated. The possibility to produce nanotube-based bolt/nut pairs and nanobearings is discussed., Comment: 29 pages, 5 figures

Structural, energetic, and tribological characteristics of double-layer graphene with commensurate and incommensurate krypton spacers of nearly monolayer coverage are studied within the van der Waals-corrected density functional theory. It is shown that when the spacer is in the commensurate phase, the graphene layers have the AA stacking. For this phase, the barriers to relative in-plane translational and rotational motion and the shear mode frequency of the graphene layers are calculated. For the incommensurate phase, both of the barriers are found to be negligibly small. A considerable change of tunneling conductance between the graphene layers separated by the commensurate krypton spacer at their relative subangstrom displacement is revealed by the use of the Bardeen method. The possibility of nanoelectromechanical systems based on the studied tribological and electronic properties of the considered heterostructures is discussed.

Reported here are the results of measurements of the resistivity (ρ) up to 300 K and of the thermoelectric power (TEP) up to 400 K of ceramic samples of the title materials. We determined also their room‐temperature lattice parameters and oxygen content as functions of Co concentrations. The metal–nonmetal transition in YBa2Cu3−xCoxOy and in EuBa2Cu3−xCoxOy is marked by the onset of the deviation of the maximum absolute TEP from a value calculated from a simple narrow‐band formula. The results indicate that the effective valency of Pr in PrBa2Cu3−xCoxOy varies from ∼3.5 at x=0 to ∼3 for x=0.5. For x=0 this material is close to the metal–nonmetal transition. In the nonmetallic regime the electrical transport is by activated hopping. In certain ranges of Co content the results are consistent with two‐band hopping conductivity with two branches, a low‐temperature and a high‐temperature branch of variable‐range‐hopping (VRH) conductivity. In the VRH regime there is a remarkable correlation between the two res...

Both van der Waals corrected density functional theory and classical calculations show that the potential relief of interaction energy between layers of graphite and few-layer graphene can be described by a simple expression containing only the first Fourier components. Thus a set of physical quantities and phenomena associated with in-plane relative vibration, translational motion and rotation of graphene layers are interrelated and are determined by a single parameter characterizing the roughness of the potential energy relief. This relationship is used to estimate the barriers to relative motion and rotation of graphene layers based on experimental measurements of shear mode frequencies., 16 pages, 1 figure

Transformation of graphene flakes to fullerenes assisted by Ni clusters is investigated using molecular dynamics simulations. The bond-order potential for Ni-C systems is developed. The potential reproduces the experimental and first-principles data on the physical properties of pure Ni as well as on relative energies of carbon species on Ni surfaces and in Ni bulk. The potential is applied for molecular dynamics simulations of the transformation of graphene flakes consisting of 50 - 400 atoms with and without Ni clusters attached. Free fullerenes, fullerenes with Ni clusters attached from outside and fullerenes encapsulating Ni clusters (Ni endofullerenes) are observed to form in the presence of Ni clusters consisting of 5 - 80 atoms. Moreover, a new type of heterofullerenes with a patch made of a Ni cluster is found to form as an intermediate structure during the transformation. The Ni clusters are shown to reduce the activation energy for the graphene-fullerene transformation from 4.0 eV to 1.5 - 1.9 eV, providing the decrease of the minimal temperature at which such a transformation can be observed experimentally from about 1400 K for free graphene flakes to about 700 - 800 K. While the transformation of free graphene flakes is found to occur through formation of chains of two-coordinated carbon atoms at the flake edges, the mechanism of the Ni-assisted graphene-fullerene transformation is revealed to be based on the transfer of carbon atoms from the graphene flake to the Ni cluster and back. The way of controlled synthesis of endofullerenes with a transition metal cluster inside and heterofullerenes with a transition metal patch is also proposed., Comment: 39 pages, 8 figures

Temperature-activated diffusion of a graphene flake on a graphene layer and a commensurate-incommensurate phase transition in bilayer graphene are investigated using the classical potential for graphene developed recently on the basis of first-principles calculations with the van der Waals correction. It is shown that rotation of graphene flakes to incommensurate states can significantly contribute to diffusion of the flakes consisting of several tens of atoms even at room temperature. Formation of incommensurability defects in bilayer graphene upon stretching of one of the layers is observed by molecular dynamics simulations.

The self-retracting motion of a graphene flake on a stack of graphene flakes is studied using molecular dynamics simulations. It is shown that in the case when the extended flake is initially rotated to an incommensurate state, there is no barrier to the self-retracting motion of the flake and the flake retracts as fast as possible. If the extended flake is initially commensurate with the other flakes, the self-retracting motion is hindered by potential energy barriers. However, in this case, the rotation of the flake to incommensurate states is often observed. Such a rotation is found to be induced by the torque acting on the flake on hills of the potential relief of the interaction energy between the flakes. Contrary to carbon nanotubes, telescopic oscillations of the graphene flake are suppressed because of the high dynamic friction related to the excitation of flexural vibrations of the flake. This makes graphene promising for the use in fast-responding electromechanical memory cells., Comment: 24 pages, 8 figures

A commensurate-incommensurate phase transition in bilayer graphene is investigated in the framework of the Frenkel-Kontorova model extended to the case of two interacting chains of particles. Analytic expressions are derived to estimate the critical unit elongation of one of the graphene layers at which the transition to the incommensurate phase takes place, the length and formation energy of incommensurability defects (IDs) and the threshold force required to start relative motion of the layers on the basis of dispersion-corrected density functional theory calculations of the interlayer interaction energy as a function of the relative position of the layers. These estimates are confirmed by atomistic calculations using the DFT-D based classical potential. The possibility to measure the barriers for relative motion of graphene layers by the study of formation of IDs in bilayer graphene is discussed., 22 pages, 4 figures

Diffusion and drift of a graphene flake on a graphite surface are analyzed. A potential energy relief of the graphene flake is computed using ab initio and empirical calculations. Based on the analysis of this relief, different mechanisms of diffusion and drift of the graphene flake on the graphite surface are considered. A new mechanism of diffusion and drift of the flake is proposed. According to the proposed mechanism, rotational transition of the flake from commensurate to incommensurate state takes place with subsequent simultaneous rotation and translational motion until a commensurate state is reached again, and so on. Analytic expressions for the diffusion coefficient and mobility of the flake corresponding to different mechanisms are derived in wide ranges of temperatures and sizes of the flake. The molecular dynamics simulations and estimates based on ab initio and empirical calculations demonstrate that the proposed mechanism can be dominant under certain conditions. The influence of structural defects on the diffusion of the flake is examined on the basis of calculations of the potential energy relief and molecular dynamics simulations. The methods of control over the diffusion and drift of graphene components in nanoelectromechanical systems are discussed. The possibility to experimentally determine the barriers to relative motion of graphene layers based on the study of diffusion of a graphene flake is considered. The results obtained can be also applied to polycyclic aromatic molecules on graphene and should be qualitatively valid for a set of commensurate adsorbate-adsorbent systems., Comment: 42 pages, 7 figures